The reduction of NOx has long been a concern in the field of fossil fuel combustion, particularly with the operation of modern power plants. Oxides of nitrogen are a byproduct of the combustion of hydrocarbon fuels, such as pulverized coal, gas, or oil. One typical post-combustion process for lowering the NOx emissions is that of Selective Catalytic Reduction (SCR). SCR systems use catalyst and a reactant such as ammonia gas to dissociate NOx to molecular nitrogen and water vapor.
The reduction of NOx has long been a concern in the field of fossil fuel combustion, particularly with the operation of modern power plants. Oxides of nitrogen are a byproduct of the combustion of hydrocarbon fuels, such as pulverized coal, gas, or oil. One typical post-combustion process for lowering the NOx emissions is that of Selective Catalytic Reduction (SCR). SCR systems use a catalyst and a reactant such as ammonia gas to disassociate NOx to molecular nitrogen and water vapor.
Ducts are constructed to transport air and flue gas, separately, or in combination. Combustion of hydrocarbon fuels occurs within the boiler, creating hot flue gases that are directed through an exit to a flue or gas duct. Ammonia as a reactant is introduced into the same flue or gas duct via sparger tubes which form an ammonia injection grid (AIG) and spray ammonia. The ammonia is mixed with the hot flue gases. The ammonia/flue gas mixture then enters the SCR chamber wherein catalytic reductions take place between the ammonia/flue gas mixture and the catalytic material.
After leaving the SCR, the ammonia/flue gas mixture travels to an air preheater followed by a flue gas cleaning system such as a scrubber and then to the atmosphere via a stack.
At some point before the ammonia reaches the SCR, it is vaporized. Various systems are used to supply the heat necessary for ammonia vaporization. Many of these systems are based on heating the dilution air and then using this heat to vaporize the ammonia prior to the AIG. Typically, these systems range from heating with direct fired natural gas to heating with a steam coil or electric heater. U.S. Pat. No. 5,296,206 describes a heat exchanger being used to heat the diluted air while extracting heat from the flue gas between the boiler exit and the SCR chamber inlet. The heated dilution air is then passed to a location outside the flue gas path where it is used to vaporize the ammonia entering the gas duct through the AIG.
Use of an external heat source to vaporize ammonia is disadvantageous as described in the background section of U.S. Pat. No. 5,437,851. There is a need in the fossil fuel burning industry for a means of heating dilution air in which no external energy source is needed for the heating. There is also a need for a location for the means for heating that has relatively clean air thus minimizing concerns for deposition, fouling, corrosion, and erosion.